In recent years, nitride semiconductor light emitting devices such as a blue light emitting diode (LED), a laser diode (LD) or the like, using nitride semiconductor, have been in practical use. On the other hand, a ZnO based compound is superior to a GaN based compound (which means, besides GaN, a compound in which a part or all of Ga is substituted with other element of group III element and the same applies hereinafter) in emitting light in a range of a short wavelength. Concretely, that is because an exciton of ZnO, which is formed by recombination of a hole and an electron in a solid, is stable even at a room temperature because of having a large binding energy of 60 meV (GaN has that of 24 mev). Thus, the ZnO based compound is expected for a light emitting device, a light receiving device or the like of a blue or ultraviolet region, in place of GaN, however, as it is known that crystal defects or the like occur by oxygen vacancies or Zn atoms between lattices in the ZnO based compound, the ZnO based compound becomes to have normal n-type conductivity because electrons not contributing are generated by the crystal defects, and it is necessary to lower concentration of the remained electrons to form ZnO based compound of p-type conductivity.
Concretely, although a sapphire substrate is generally used, a principal plane of which is a C plane, in order to form a semiconductor device using ZnO based compound, ZnO based compound semiconductor layers on the sapphire substrate are usually grown in a direction of −c axis (oxygen plane). However, in the ZnO based compound semiconductor layers formed by crystal growth in the direction of −c axis, since a doping effect of nitrogen of a p-type dopant depends strongly on temperature, temperature of the substrate is required to be lowered in order to dope with nitrogen. If the temperature of the substrate is lowered, crystallinity becomes to be lowered, centers compensating acceptors are introduced, and nitrogen can not be activated after all, thereby the p-type ZnO based compound semiconductor layers with sufficiently high carrier concentration can not be obtained (cf. for example NON-PATENT DOCUMENT 1). In addition, there is known a method of temperature modulation in which the p-type ZnO based compound semiconductor layers are grown by varying the temperature between 400 and 1,000° C. reciprocally, utilizing the temperature dependence, thereby the p-type layers with sufficiently high carrier concentration being obtained (cf. for example NON-PATENT DOCUMENT 2). However, since expansion and contraction are repeated by continuous repetition of heating and cooling, a large load is applied to an apparatus, a scale of the apparatus becomes large, and a period for maintenance becomes shorter.
On the other hand, the present inventors studied other methods for forming the p-type ZnO based compound semiconductor layers with high carrier concentration, and, as a result, it was found and already disclosed that, on a ZnO substrate (Zn plane) or a sapphire substrate in which a principal plane is a C plane and orientated to +c axis, a GaN layer orientated to +c axis is formed as a base layer, and ZnO based compound semiconductor layers orientated to the same direction, namely to +c axis, are laminated thereon, thereby c axis orientations of the substrate, the GaN layer of the base layer and the ZnO based compound semiconductor layers are arranged to be equal, excellent crystallinity is maintained, and the p-type ZnO based compound semiconductor layers with high carrier concentration can be formed (cf. for example PATENT DOCUMENT 1).
PATENT DOCUMENT 1: Japanese Patent Application Laid-Open No. 2004-304166
NON-PATENT DOCUMENT 1: Journal of Crystal Growth 237-239 (2002)503
NON-PATENT DOCUMENT 2: Nature Material vol. 4 (2005) p. 42